Myxozoans in fish

Myxozoans are commonly found parasites that infect both marine and freshwater fishes. They are found principally in the muscle, the brain and the gall bladder of their host fishes.

Fish filet with obvious white cysts, in this case a sign of infection with the parasite Kudoa.

Some species of myxozoans can cause death in fish. Non-lethal effects can include the production of small but obvious white cysts in the muscle that make fillets unsightly, unappetising and therefore unmarketable. Some species in the genus Kudoa produce enzymes after their host dies that cause muscle breakdown, making fillets soft and watery. This is known as post mortem muscle liquefaction. Stained histological section of fish muscle showing numerous Kudoa parasites.

These types of myxozoan infections pose significant challenges in commercial fisheries and are also of concern for aquaculture and Australia's biosecurity.

Taxonomic studies of myxozoans at the Queensland Museum have revealed that they are likely to be far more diverse than their fish hosts. For example, a study of Great Barrier Reef fishes with the parasite Ceratomyxa, found mostly in the gall-bladder and just one of the many genera of the Phylum Myxozoa, has shown that there is likely to be almost one species of parasite for every species of fish. This means that Australia's coral reef fish fauna could harbour over 1,500 Ceratomyxa species, of which only 30 (or about than 2%) have actually been discovered by scientists.

Thus, this group is exceptionally rich in coral reef fishes but remains exceptionally poorly known. Knowledge of the identity, range of hosts and geographical distribution of these parasites will increase our understanding of patterns of infection and have specific application when they affect wild harvest and aquaculture fisheries.

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